Dielectric wave guide



Sept. 27, 1966 M. P. PRACHE 3,275,956

DIELECTRIC WAVE GUIDE Filed July 15, 1964 United States Patent 3,275,956 DIELECTRIC WAVE GUIDE Marie Pierre Prache, Versailles, France, assignor to Lignes Telegraphiques & Telephoniques, Paris, France Filed July 15, 1964, Ser. No. 382,906 Claims priority, application France, Aug. 6, 1963, 943,906 4 Claims. (Cl. 33395) The present invention relates to transmission lines for electromagnetic waves in the VHF spectrum, which lines essentially comprise a circular cylindrical tube made of very low-loss dielectric material.

The construction of the transmission lines forming the subject of the present invention may comprise:

(a) tubular structures composed exclusively of dielectric materials;

(b) structures as defined in (a), with the addition of an outer metallic tubular conductor;

(c) structures as defined in (b), with the addition of an inner metallic conductor.

It is known that in a structure of one of the types (a), (b), or (c), it is possible, at sufficiently high frequencies, to effect propagation of TE waves or TM waves.

It is also known that in lines of this sort, it is desirable that transmission should take place using a fundamental TM or TE mode which has circular symmetry about the tube axis, and that there should be only one such mode. Consequently, the accidental appearance of other modes must be prevented. In other words, the use of carrier currents in telecommunications circuits requires that all waves of one frequency be propagated at the same velocity. Once such a mode of propagation has been selected, the aim should be to make its transmission preferential to the transmission of other modes which could be propagated in the same direction at different velocities. The higher order modes having circular symmetry are of no concern since their cut-off frequencies are much higher than the cut-off frequencies of the fundamental modes of rotation. Modes do, however exist, which do not have circular symmetry and whose cut-off frequency is of the same order of magnitude, as the cut-off frequency of one of the said fundamental modes or even lower than this. In the ensuing description, the TM type fundamental mode of rotation will be abbreviated to circular TM mode and the TE fundamental mode of rotation to circular TE mode.

The present invention contemplates the provision of devices adapted to favor the propagation of a selected one of the TE and TM circular modes, the other mode (whichever it happens to be) being rejected along with those modes which are not of rotational type.

In the ensuing description, cylindrical co-ordinates will be employed, z for the longitudinal axis of the tube, this being the direction of propagation of the waves, r for the distance of a point from said axis and p for the angle, about this axis, defining the location of a radial vector.

The components of the electric field in the longitudinal, radial and circular directions will be designated by E E and B, respectively.

The operation of the devices to which the invention relates, is based on the fact that the TM or IE circular modes are such that throughout propagation, one at least of the components of their electric fields is zero (E for the TB mode and E, for the TM mode); this component will not be zero for any of the non-circular modes. The aim of the invention is to make more difficult the propagation of those waves which contain this component in order to favor propagation of the mode for which the said component should be zero.

In accordance with the invention, a transmission line is provided which is adapted to a predetermined one of the fundamental modes TE and TM having circular symmetry about the axis of said line, the line comprising a circular, cylindrical, dielectric tube made of materials presenting zones of alternately higher and lower permittivity the separating surfaces between these zones being perpendicular to a direction in which the electric field component is normally zero for the said predetermined mode; the electric field in this same direction is not zero for modes other than the said predetermined mode.

In a first embodiment of the invention, the selected mode of propagation is the TM mode of rotation for which the circular component E, of the electric field is zero, the zones of lower permittivity in the dielectric tube being separated from those of higher permittivity by planes passing through the longitudinal axis of the tube; the tube is formed from longitudinal elements in the shape of cylindrical sections and made of dielectric material of fairly high permittivity, the transverse sectional extent of these sections being limited by two circumferential arcs having the same center and being also limited by straight-ended segments arranged on lines passing through the common center, the said sections being separated from one another by strips of material of lower permittivity. The apparent permittivity is thus much lower in the circular direction than in the longitudinal direction, which tends to prevent propagation of the undesired TE mode.

In a second embodiment of the invention, the mode of propagation selected is the TE circular mode, for which the longitudinal component E and the radial component E of the electric field are both zero, the zones of lower permittivity in the dielectric tube being separated from the zones of higher permittivity either by planes perpendicular to the longitudinal axis of the tube or by cylindrical surfaces the axes of which coincide with the longitudinal axis of the tube, or by both these means simultaneously. In this embodiment, the dielectric tube consists of rings of dielectric material of fairly high permittivity, alternating with rings of dielectric material of lower permittivity, these latter also being shorter in the longitudinal direction than the first rings.

The advantage of the proposed arrangements is easily understood if, considering the reduction in the mean permittivity of the dielectric tube, which reduction, thanks to the anisotropic nature of the permittivity resulting ice - ,from the non-homogeneous structure of the tube, is only effective for the undesired modes, We note that the cut-off frequency of these latter modes is increased and the likelihood of them appearing and being propagated thus reduced.

In the above mentioned embodiments, it is not necessary for the zones of lower permittivity to be occupied by a homogeneous material. For instance, they may be air-gaps, in which case means are provided for mechanically supporting the sections of dielectric material exhibiting the higher permittivity.

Variations of the invention can also be employed in cases where the transmission line also incorporates a cylindrical metallic tube outside the dielectric tube or where it has both an outer metallic tube of this type and an inner cylindrical metallic tube.

The invention will be described making reference to exemplary embodiments which are given by way of non-limitative example in the appended drawing, wherein:

FIGURES 1 and 2 geometrically illustrate the components of the magnetic and electric fields for the TM and TE, circular modes respectively.

FIGURE 3 represents a transmission line of the type in accordance with the invention, for the TM circular mode.

FIGURE 4 represents transmission lines in accordance with the invention, for the TE circular mode.

FIGURE represents a transmission line such as that illustrated in FIGURE 3, but with an outer metal tube surrounding the dielectric tube.

FIGURE 6 illustrates a transmission line such as that illustrated in FIGURE 3, but with outer and inner tubular conductors associated wit-h the dielectric tube.

For the TM mode, the components of the magnetic and electric field which are not Zero are H and E E as illustrated in FIGURE 1. The E,, component is zero.

For the TE mode, the electric and magnetic field components which are not zero are H H and E as illustrated in FIGURE 2. The components E and E are zero.

For those modes which are not of rotational type to be propagated, none of the six components E E,,

E,, H H,, H, should be zero.

FIGURE 3 is a transverse section of a transmission line in accordance with the invention, comprising a dielectric tube designed to favor the propagation of the TM mode. This tube is constituted by a certain number of principal longitudinal elements in the form of sections of cylinders 1, separated by auxiliary strips 2 whose planes of symmetry pass through the tube axis. The elements 1 are made ofdielectric material of quite high permittivity and the strips 2 of dielectric material of lower permittivity, foamed plastic for instance.

The number of longitudinal elements 1 can, of course, differ from that illustrated in the figure and the elements need not all be identical.

The presence of the strips 2 of low permittivity, makes propagation of waves containing the component B, more difficult and favors propagation of the TM mode for which E, is zero.

In FIGURE 4, which is a sec-tion taken on a plane passing through the longitudinal axis of symmetry, a transmission line is illustrated which comprises a dielectric tube designed to make difiicult the propagation of waves containing an electric field component E and thus to favor transmission of the TB mode for which this component is zero. The tube is built up of rings 3 of dielectric material exhibiting fairly high permittivity, and of rings 4 the length of which in the direction of the tube axis is shorter than that of the rings 3 and which are made of a dielectric material exhibiting lower permittivity (foamed plastic for example) than the material of the rings 3.

FIGURE 5 is a transverse section through a transmission line in accordance with the invention, in which a metal cylinder 7 is disposed outside a dielectric tube similar to that illustrated in FIGURE 3. The transmission line is thus a Wave guide constituted by a cylindrical conductor tube having inside it a cylindrical dielectric tube of the type in accordance with the invention.

FIGURE 6 is a transverse section through a transmission line of the type in accordance with the invention, in which a cylindrical conductor tube 8 is disposed outside a dielectric tube similar to that illustrated 4 in FIGURE 3, and in which a cylindrical metal conductor 9 is provided inside the dielectric tube. The transmission line thus constitutes a coaxial line between the inner and outer conductors of which is situated a dielectric tube of the type in accordance with the invention.

The means for spacing the metallic and dielectric elements away from one another are not illustrated in FIGURES 5 and 6, simply in order to prevent confusion.

The invention can be applied equally well to cases in which, as in FIGURES 5 and -6, the dielectric tube of FIGURE 3 is replaced by a dielectric tube of the type illustrated in FIGURE 4.

What is claimed is:

1. A transmission line for very high frequency electromagnetic waves, adapted to the propagation of the fundamental TM mode having revolution symmetry about a longitudinal axis of said line, comprising a circular cylindrical tube of low-loss dielectric material, in which said dielectric tube comprises principal elongated elements the length of which extends in a direction parallel to said axis and each having their transversal cross-section limited by two circular arcs centered on said axis and two straight line segments passing through said axis, said principal elements being separated from each other by auxiliary strips having smaller cross-section than the principal elongated elements and constituted of a dielectric material having a permittivity lower than that of the material constituting said elements.

2. A transmission line as claimed in claim 1, further comprising a cylindrical tube constituted of an electrically conductive material arranged about said dielectric tube.

3. A transmission line as claimed in claim 2, further comprising a cylindrical electric conductor inside said dielectric tube with the longitudinal axis of said conductor substantially coinciding with that of said tube.

4. A transmission line for very high frequency electromagnetic waves, adapted to the propagation of the fundamental TE mode having revolution symmetry about a longitudinal axis of said line, comprising a circular cylindrical tube of low-loss dielectric material, in which said dielectric tube is made up from rings alternately consisting of dielectric materials having a higher and a lower permittivity, said rings being stacked in the direction of said longitudinal axis and the length along said direction of the lower permittivity rings being shorter than that of the higher permittivity rings.

References Cited by the Examiner UNITED STATES PATENTS 2,790,149 4/ 1957 Robertson 333-- 3,040,278 6/1962 Griernsmann 33395 FOREIGN PATENTS 1,047,897 12/ 1958 Germany.

HERMAN KARL SAALBACH, Primary Examiner. L. ALLAHUT, Assistant Examiner. 

1. A TRANSMISSION LINE FOR VERY HIGH FREQUENCY ELECTROMAGNETIC WAVES, ADAPTED TO THE PROPAGATION OF THE FUNDAMETAL TM MODE HAVING REVOLUTION SYMMETRY ABOUT A LONGITUDINAL AXIS OF SAID LINE, COMPRISING A CIRCULAR CYLINDRICAL TUBE OF LOW-LOSS DIELECTRIC MATERIAL, IN WHICH SAID DIELECTRIC TUBE COMPRISES PRINCIPAL ELONGATED ELEMENT THE LENGTH OF WHICH EXTENDS IN A DIRECTION PARALLEL TO SAID AXIS AND EACH HAVING THEIR TRANSVERSAL CROSS-SECTION LIMITED BY TWO CIRCULAR ARCS CENTERED ON SAID AXIS AND TWO STRAIGHT LINE SEGMENTS PASSING THROUGH SAID AXIS, SAID PRINCIPAL ELEMENTS BEING SEPARATED FROM EACH OTHER BY AUXILIARY STRIPS HAVING SMALLER CROSS-SECTION THAN THE PRINCIPAL ELONGATED ELEMENTS AND CONSTITUTED OF A DIELECTRIC MATERIAL HAVING A PERMITTIVITY LOWER THAN THAT OF THE MATERIAL CONSTITUTING SAID ELEMENTS.
 4. A TRANSMISSION LINE FOR VERY HIGH FREQUENCY ELECTROMAGNETIC WAVES, ADAPTED TO THE PROPAGATION OF THE FUNDAMENTAL TE MODE HAVING REVOLUTION SYMMETRY ABOUT A LONGITUDINAL AXIS OF SAID LINE, COMPRISING A CIRCULAR CYLINDRICAL TUBE OF LOW-LOSS DIELECTRIC MATERIAL, IN WHICH SAID DIELECTRIC TUBE IS MADE UP FROM RINGS ALTERNATELY CONSISTING OF DIELECTRIC MATERIALS HAVING A HIGHER AND A LOWER PERMITTIVITY, SAID RINGS BEING STACKED IN THE DIRECTION OF SAID LONGITUDINAL AXIS AND THE LENGTH ALONG SAID DIRECTION OF THE LOWER PERMITTIVITY RINGS BEING SHORTER THAN THAT OF THE HIGHER PERMITTIVITY RINGS. 